A clutch unit for transport machines is equipped with a wet multiple-plate clutch mechanism. The wet multiple-plate clutch mechanism has a clutch drum which is nearly in the shape of a cup. In the clutch drum, an annular coned disk spring is provided between a driven plate and a piston, the driven plate being provided proximate to a bottom surface of the clutch drum so as to be movable in an axial direction (see Japanese Unexamined Patent Application Publication No. 9-32918). The coned disk spring is disposed such that an inner peripheral edge portion of the coned disk spring is supported by the piston and a peripheral edge portion thereof is supported by the driven plate proximate to the bottom surface. The coned disk spring elastically deforms so that the shape thereof changes from a shape which is similar to that of a dish to one which is nearly flat, thereby absorbing shocks which occur in the engaging action of the clutch mechanism.
In properties of the above coned disk spring, a load (hereinafter referred to as “flat deformation time load”), which is generated in the coned disk spring when the coned disk spring deforms to become nearly flat by elastic deformation (that is, when displacement amount of the coned disk spring reaches the stroke distance ST defined by the difference between the height H and the thickness T as shown in FIG. 6), is necessary as a design parameter of the clutch mechanism. The flat deformation time load depends on the outer diameter, inner diameter, thickness T and stroke distance ST of the coned disk spring. The outer diameter, the inner diameter, and the stroke distance ST are determined as design parameters beforehand. Therefore, the flat deformation time load is controlled by the thickness T.
In clutch units for CVT (Continuously Variable Transmission) vehicles and AT (Automatic Transmission) vehicles, there is a clutch unit equipped with plural clutch mechanisms which are different from each other in size and have the same axis. In each clutch mechanism, a coned disk spring, which has an outer diameter corresponding to clutch drum thereof, is disposed. In this case, since flat deformation time loads of the coned disk springs are not generally equal to each other, the respective coned disk springs are produced from materials which are different from each other in thickness.
However, materials, from which blanks of the coned disk springs have been punched, cannot be used, and they are thereby scraped. In the production of the coned disk springs, amount of scrapped materials is very large. Due to this, the material yield of the coned disk spring is low, and the production cost is thereby very high.